1. Field of the Invention
The present invention relates to a Cathode Ray Tube (CRT) monitor, and more particularly, to a circuit for controlling a delay of a dynamic focus signal in a CRT monitor.
2. Description of the Related Art
In general, an electron beam emitted from an electron gun has different driving distances to a central portion and an edge portion of a screen. As a result, the focus is not uniform over the entire screen. In particular, the focus at a central portion of the screen is different than the focus at an edge portion of the screen. To overcome this problem, a dynamic focus circuit, which applies different focus voltages to the central portion and to the edge portion of the screen, is required. A signal that is generated by the dynamic focus circuit is called a dynamic focus signal.
A dynamic focus signal is amplified to a relatively high voltage in order to drive a CRT monitor, but the dynamic focus signal is sometimes delayed in the amplification process.
Various kinds of video modes are used for a CRT monitor. Each video mode uses a different frequency. However, in a case where a dynamic focus signal is amplified, a delay time due to the amplification is always the same for all the video modes. Therefore, a complete focusing operation cannot be performed. That is, since a delay time (the delay time is below about 1 us) generated in the process where the dynamic focus signal is amplified is applied to each of the video modes at a different ratio, it is necessary to compensate for a delay time in each video mode individually. This can be a serious problem.
FIG. 1 is a block diagram showing a delay control circuit for controlling delays in a conventional dynamic focus signal. The conventional delay control circuit 100 includes a horizontal deflection transformer 110, a shaping pulse generating circuit 120 and a selection circuit 130. The horizontal deflection transformer 110 generates a fly-back signal (SFB). The fly-back signal (SFB) controls the location of an electronic beam on a CRT monitor. The electronic beam moves from the left to the right of the screen in order to output information on the screen of the CRT monitor. When the electronic beam moves to the end of the screen, it moves to the starting point of the screen in order to output the next screen. The signal which controls these movements is referred to as a fly-back signal (SFB).
The fly-back signal (SFB) generated by the horizontal deflection transformer 110 cannot have a shaped pulse form but is generated in a uniformly shaped pulse waveform by the shaping pulse generating circuit 120.
The selection circuit 130 selects one of the rising edge or the center point of the fly-back signal (SFB1) with a shaping pulse form in response to a selection signal (SEL) and generates a delay control signal (DECTRLS).
FIG. 2 is a block diagram of a circuit which generates a dynamic focus signal (DFS) in response to the delay control signal (DECTRLS) of FIG. 1.
A mono stable circuit 210 converts an input signal into a pulse form. When the delay control signal (DECTRLS) is converted into a pulse waveform by the mono stable circuit 210 and is input to a serration wave generating circuit 220, the serration wave generating circuit 220 generates serration waves (STS) to generate a dynamic focus signal. The serration wave generating circuit 220 is charged by receiving current from the exterior and is discharged when the delay control signal (DECTRLS) is input. The serration wave generating circuit 220 is discharged by the delay control signal (DECTRLS) and is charged by receiving current from the exterior when a voltage reaches a certain reference level. In this way, a serration wave signal (STS) is output.
The dynamic focus signal generating circuit 230 receives the output signal (STS) of the serration wave generating circuit 220 and generates a dynamic focus signal (DFS) having the form of a parabola. The dynamic focus signal (DFS) can be generated by squaring the output serration wave signal (STS) of the serration wave generating circuit 220.
FIG. 3 is a waveform diagram showing a relationship between the fly-back signal and the dynamic focus signal.
FIG. 3(i) shows the serration wave signal (STS) and the dynamic focus signal (DFS) in a case where the center of the shaped fly-back signal (SFB1) is selected. FIG. 3(ii) shows the serration wave signal (STS) and the dynamic focus signal (DFS) in a case where the rising edge of the shaped fly-back signal (SFB1) is selected.
As shown in FIG. 3, only a center or a rising edge of the fly-back signal (SFB1) is selected as a point at which the dynamic focus signal (DFS) is generated. Therefore, delays which occur when the dynamic focus signal (DFS) is amplified cannot be compensated for accurately.
To solve the above-described problems, it is an object of the present invention to provide a dynamic focus signal delay control circuit for controlling delays generated in the process where a dynamic focus signal of a CRT monitor is amplified.
It is another object of the present invention to provide a dynamic focus signal delay control method for controlling delays generated in the process where a dynamic focus signal is amplified in a CRT monitor.
To achieve the above object of the present invention, a delay control circuit according to a first embodiment of the present invention includes a pulse center detecting circuit, a first delay control signal generating circuit, and a selection circuit.
The pulse center detecting circuit generates a center detecting signal that indicates a pulse center of a first pulse signal. The first delay control signal generating circuit generates a first delay control signal having a first logic level or a second logic level depending on the result of a comparison of a first comparative signal level generated in a form of a first degree function in response to the first pulse signal with a predetermined second comparative signal level. The selection circuit selects one of the center detecting signal and the first delay control signal in response to a predetermined selection signal and generates the selected signal as a second delay control signal that controls an amount of delay time of the dynamic focus signal.
Preferably, the first delay control signal generating circuit includes a first comparative signal generating circuit and a comparative circuit. The first comparative signal generating circuit responds to the first pulse signal, recognizes a rising edge of the first pulse signal and generates the first comparative signal in a form of a first-degree function. The comparative circuit compares the level of the first comparative signal with that of the second comparative signal. If the level of the first comparative signal is larger than that of the second comparative signal, the comparative circuit generates the first delay control signal having the first logic level. On the other hand, if the level of the first comparative signal is smaller than that of the second comparative signal, the comparative circuit generates the first delay control signal having the second logic level.
The first comparative signal generating circuit includes a charge/discharge circuit which converts the first pulse signal into a current signal in response to the first pulse signal, and a capacitor which generates the first comparative signal with a triangular waveform by using an output current of the charge/discharge circuit.
The second comparative signal is a direct current signal which can be changed between the minimum and maximum levels of the first comparative signal. The center detecting signal is a signal having a phase which is changed from a low level to a high level or from a high level to a low level at a pulse center of the first pulse signal. The first pulse signal is a fly-back signal that controls the position of an electronic beam on a monitor.
The delay control circuit further includes a digital-to-analog converter that generates the second comparative signal. A point at which the first delay control signal is generated is controlled in response to changes of the second comparative signal level.
To achieve the above object of the invention, a circuit for controlling a delay of a dynamic focus signal, according to a second embodiment of the present invention, includes a first comparative signal generating circuit and a comparative circuit.
The first comparative signal generating circuit responds to the first pulse signal, recognizes a rising edge of the first pulse signal and generates the first comparative signal in a form of a first-degree function. The comparative circuit generates a delay control signal having a first logic level or a second logic level depending on the result of comparison of the first comparative signal level with a predetermined second comparative signal level.
Preferably, the first comparative signal generating circuit includes a charge/discharge circuit which converts the first pulse signal into a current signal in response to the first pulse signal, and a capacitor which generates the first comparative signal with a triangular waveform by using an output current of the charge/discharge circuit.
The second comparative signal is a direct current signal which can be changed between the minimum and maximum levels of the first comparative signal. The delay control circuit compares the first comparative signal level and the second comparative signal level, and if the first comparative signal level is larger than the second comparative signal level, the delay control signal is generated in the first logic level, and if the first comparative signal is smaller than the level of the second comparative signal, the delay control signal is generated in the second logic level, and a point at which the signal is generated is controlled in response to changes of the second comparative signal level.
The delay control circuit of a dynamic focus signal further includes a digital-to-analog converter that generates the second comparative signal. The first pulse signal is a fly-back signal that controls the position of an electronic beam on a monitor.
To achieve the second object of the present invention, there is provided a method for controlling an amount of delay of a dynamic focus signal in a CRT monitor. In this method, first, a first comparative signal having a form of a first degree function is generated by responding to a predetermined first pulse signal and recognizing a rising edge of the first pulse signal. A predetermined second comparative signal is received and compared with the first comparative signal level. If the first comparative signal level is larger than the second comparative signal level, a first delay control signal having the first logic level is generated. But, if the first comparative signal level is smaller than the second comparative signal, a first delay control signal having the second logic level is generated. The dynamic focus signal whose delay is controlled in response to the results of the first delay control signal is generated.
In the first comparative signal generation step, the first pulse signal is converted into a current signal in response to the first pulse signal, and the first comparative signal with a triangular waveform is generated by charging/discharging the current signal.
The second comparative signal is a direct current signal that changes between the minimum and maximum levels of the first comparative signal. The points at which first delay control signal is generated are controlled in response to changes of the second comparative signal level. The first pulse signal is a fly-back signal that controls the position of an electronic beam on a monitor.
In the method according to the present invention, furthermore, a center detecting signal which detects a pulse center of the first pulse signal is generated. Either the center detecting signal or the first delay control signal is selected in response to the selective signal, and the selected signal is output as the second delay control signal. The dynamic focus signal whose amount to be delayed is controlled in response to the second delay control signal is generated.
In the circuit and method for controlling a delay of a dynamic focus signal, an amount of delay time in the process where a dynamic focus signal is amplified in a CRT monitor is controlled.